This invention relates to a method of preparing a polymer-coated particulate inorganic material. Graphite fluoride is a typical example of inorganic materials to which the invention is applicable. A polymer-coated inorganic material obtained by the method of the invention is of use either in powder form, as solid lubricant for example, or in compacted form optionally with addition of a synthetic resin as bearings or sealing elements for example.
Powders of various inorganic materials are of wide application and have been used, for example, as solid lubricants, as materials for press-formed articles and fillers for plastics and rubbers. In some cases there arises the need of subjecting the particles of the inorganic material to certain treatment or using a certain auxiliary material in order to obviate some disadvantages or inconveniences attributed to physical and chemical properties of the particles surfaces. For example, fine particles of an inorganic material used as solid lubricant are often dispersed in lubricating oil or grease, but usually it is difficult to realize uniform dispersion because of low affinity of the inorganic material for the dispersing medium and a great difference in specific gravity between the inorganic material and the dispersing medium and therefore, it is usual to add a relatively large amount of dispersing agent. Although the dispersion is improved, the presence of a large amount of dispersing agent becomes an obstacle to efficient action of the dispersed solid lubricant, and therefore often it becomes necessary to use an increased quantity of solid lubricant in order to achieve a desired purpose.
Typical examples of inorganic materials useful as solid lubricants are graphite, graphite fluoride, molybdenum disulfide, tungsten disulfide, boron nitride, talc and mica. Among these materials graphite fluoride has been attracting increasing interest mainly by reason of its remarkably high lubricating ability, but it is very difficult to well disperse pure graphite fluoride particles in water or organic medium.
Graphite fluoride is a solid material in the form of white or grayish powder obtained by reaction between graphite or carbon in different form and fluorine. As typical examples of graphite fluoride, (CF).sub.n and (C.sub.2 F).sub.n are known as stable and industrially useful polymeric compounds. Generally graphite fluoride exhibits remarkably high lubricating and water- and oil-repelling properties and is excellent in resistance to various chemicals. Accordingly graphite fluoride has been used as solid lubricant in many fields and, besides, serves for releasing, water- or oil-repelling and anti-contaminating purposes. Also it is known to produce a solid body of a specific use, such as an electrolytic cell electrode, by press-shaping of a composition containing graphite fluoride as a main ingredient.
In practical applications, however, very strong water- and oil-repelling property of graphite fluoride, which is attributed to extraordinarily low surface energy of this material, offers inconvenience or difficulty in various respects. That is, this material can hardly be dispersed in water and is very low in miscibility with organic materials and poor in formability.
Regarding the use of graphite fluoride as solid lubricant, it is ideal that fine particles of pure graphite fluoride provide a continuous and closely contacting film on the applied surface, and for this reason often it is wished to disperse graphite fluoride in water without using any auxiliary material. Actually, however, graphite fluoride is practically devoid of wettability with water as demonstrated by the fact that the contact angle of (CF).sub.n for water is 145.degree., which is a very large value compared with the 100.degree.-110.degree. contact angle of polytetrafluoroethylene (PTFE) useful as solid lubricant, and therefore it is practically impossible to disperse pure graphite fluoride in water.
In view of this problem, it has been proposed to use a dispersing agent such as colloidal silica jointly with graphite fluoride. Although the use of such a dispersing agent is effective for preparation of an aqueous dispersion, there arises another problem that the content of graphite fluoride in the dispersed solid phase cannot be made so large as desired: the graphite fluoride content must be limited to about 60% by weight at the maximum. Therefore, it becomes impossible to fully utilize the favorable properties of graphite fluoride originated in the low surface energy of this material. Also it has been proposed to coat the particles of graphite fluoride with a binding material such as wax or a mixture of a binding material and a surface-active agent. In practice, however, it is very difficult to achieve uniform coating of the graphite fluoride particles by using a desirably small amount of such a coating material so as to allow the coated graphite fluoride to sufficiently exhibit its characteristic properties. Furthermore, the coating is not always stable under various conditions in the uses of the coated graphite fluoride because the coating is established merely by adsorption and adhesion, i.e. physical bonding, of the binding material onto the surfaces of the graphite fluoride particles.
In the case of producing an electrode of a primary cell by using graphite fluoride as a typical example of compacting of compositions containing graphite fluoride as the principal ingredient, it is known to press-shape a mixture of graphite fluoride and PTFE. The mixture is usually prepared by using an aqueous dispersion of PTFE obtained by emulsion polymerization of tetrafluoroethylene. Since graphite fluoride is strongly water-repelling, there is the need of suspending graphite fluoride particles in an organic solvent having strong affinity for water in advance of the addition of graphite fluoride to the aqueous dispersion of PTFE, and it is necessary to use a considerably large amount of organic solvent in order to fully wet the graphite fluoride particles. However, the use of such a large quantity of organic solvent causes coagulation of PTFE particles during mixing of the graphite fluoride suspended in the solvent with the aqueous dispersion of PTFE, so that the mixing results in formation of undesirably large aggromelates and fails to give a uniformly mixed powdery mixture. Furthermore, the large aggromelates are very tacky and, hence, are difficult to thoroughly pulverize. If the mixture containing the large aggromelates left uncrushed is subjected to press-shaping, it is very difficult to obtain a shaped body of good quality because the existence of the aggromelates becomes a significant obstacle to uniform transmission of the applied pressure and therefore is liable to produce strains in the press-shaped body.
Japanese patent application primary publication No. 56(1981)-112995 proposes to prepare a lubricant composition by first coating the particles of an inorganic material useful as solid lubricant with a vinylic polymer, which is formed by carrying out graft polymerization of a vinylic monomer on the surfaces of the particles, and then dispersing the polymer-coated solid inorganic material in a suitable medium. In this method, however, the efficiency of graft polymerization represented by the proportion of the polymer bonded to the inorganic material particles to the monomer subjected to the polymerization reaction hardly becomes above about 70% so that this method cannot be regarded as an efficient coating method. Furthermore, polymer-coated powders prepared by this method are not yet satisfactory in the capability of dispersing in various media or affinity for liquid materials used as dispersing media. When the powdery inorganic material to be coated by this method is low in its surface energy as in the case of graphite fluoride, it is almost impossible to realize a sufficient coating of the powder with the grafted polymer, and therefore the polymer-coated graphite fluoride still retains inconvenient properties of graphite fluoride such as low affinity for water and various organic materials and poor tendency to dispersion in various liquids.